There are two types of optical correlators. The first type is a one dimensional optical correlator which is used for correlating incoming signals with prestored signals. The second type of optical correlators is a two dimensional optical correlator which is used to correlate images. This two dimensional type correlator is well known and has been described, for example, as a passive navigation system in U.S. Pat. No. 3,636,330. The data input for the '330 system uses star field patterns. A second two dimensional optical correlator is described in U.S. Pat. No. 3,648,039 wherein one input image is translated or displaced with respect to a second reference photograph for stereo mapping purposes. The '039 system is the so called "image - to - image.revreaction. correlator. It does not use any spatial light modulator or matched filters. U.S. Pat. No. 3,494,269 describes yet another optical correlator which is used to auto or cross-correlate signals recorded on film or other appropriate media optical density variations. For the '269 system, correlation is accomplished by multiplying film records in a double-pass set up. An actual motion of at least one of the optical components is undertaken to shift or translate the projected image across the input film record. Needless to say, this is extremely cumbersome.
In the article "Real-Time Optical Correlation With Solid-State Sources.revreaction. by J.G. Duthie, et al., two configurations for a compact correlator system are disclosed. A reflective type of spatial light modulator is used here since the two configurations work in reflective modes.
This system has the following shortcoming. The Duthie setup, as shown in his FIGS. 8 and 9, uses one laser for each matched filter record. Thus, for a large array of spatially separated matched filters, an equal number of lasers is likewise needed. Furthermore, when the Duthie correlator is operated at wavelengths that are different from the one used to fabricate the matched filter, changes occur in the focal length of the transform lens, and also in the extent of the input diffraction pattern (transverse to the lens optic axis). Hence the input diffraction pattern is no longer in one-to-one correspondence with the stored matched filter pattern as is necessary for proper functioning of the system.